1. Field of Invention
The present invention relates to a method for crystallizing an amorphous silicon layer and a method for fabricating a thin film transistor (TFT) using the same, and more particularly, a method for crystallizing an amorphous silicon layer and fabricating a TFT by utilizing the Sequential Lateral Solidification (SLS) technique.
2. Discussion of the Related Art
In order to fabricate TFTs on a low heat-resistant substrate, such as a glass substrate, an amorphous silicon layer or a polycrystalline silicon layer is deposited on the substrate and etched by photolithography to form active layers for TFTs.
The mobility of charge carriers is low in the amorphous silicon layer. Accordingly, amorphous silicon TFT is not typically used as a driving circuit or a controller of a liquid crystal display (LCD). However, the mobility of charge carriers is high in the polycrystalline layer. Accordingly, polycrystalline TFTs can be used in driving circuits of a liquid crystal display (LCD), wherein devices for pixel array and driving circuits are formed simultaneously.
There are two techniques for forming polycrystalline silicon film on a glass substrate. In the first technique, an amorphous silicon film is deposited on the substrate and crystallized at a temperature of about 600.degree. C. by Solid Phase Crystallization (SPC). This technique is difficult and problematic in terms of cost and materials because it requires a very high temperature.
The second technique involves depositing an amorphous silicon film on the substrate and crystallizing the film by thermal treatment using a laser. The second technique is not a high temperature process, thus, this facilitates the formation of a polycrystalline silicon film on the glass substrate.
FIGS. 1A to 1D are schematic drawings which illustrate a method for forming a polycrystalline silicon film according to one related art. Referring to FIG. 1A, a particular region of an amorphous silicon film 10 is first irradiated at an energy density to induce formation of separated islands of amorphous silicon 11a and the liquid silicon region 11L, a region generated by the irradiation of a laser beam, completely melts.
Referring to FIG. 1B, the amorphous film is translated relative to the laser beam over a distance less than the predetermined distance for a second irradiation. While the film is translating, the liquid silicon region 11L is crystallized under low temperature through a cooling process. The separated islands of amorphous silicon 11a are used as seeds for the crystallization process which results in the growth of the liquid silicon region 11L, thereby forming a first polycrystalline silicon region 11P. Grain growth occurs not only in the middle of the growth region, but growth also occurs in the interface between the liquid silicon region 11L and solid state amorphous silicon region a-Si. The grain growth stops when the grains collide at these grain boundaries.
Referring to FIG. 1C, a selected region of a translated amorphous silicon film is secondly irradiated. Thus, separated islands of amorphous silicon 12a remain and the other portions of the silicon, namely, silicon region 12L completely melts.
In FIG. 1D, the amorphous film is translated relative to the laser beam for the next irradiation. While the film is translating, the liquid silicon region 12L is crystallized under a low temperature cooling process. The separated islands of amorphous silicon 12a are used as seeds which grow into the liquid silicon region 12L, thereby forming a second polycrystalline silicon region 12P. Moreover, grain growth occurs at the interface between the liquid silicon region 12L and solid state amorphous silicon region a-Si, as well as at the interface between the liquid silicon region 12L and the first polycrystalline silicon region 11P.
The above described processes of irradiating and crystallizing are repeated over a total translation distance in order to crystallize the entire film. However, since the size of each silicon grain is not uniform and the location of the grain boundary varies in the polycrystalline silicon layer, a device-to-device uniformity is degraded in TFTs fabricated by such methods.
Accordingly, it is desirable and necessary to make the location of the grain boundary uniform and the grain size large.
Accordingly, it is proposed that a polycrystalline silicon film be formed on the glass substrate by using Sequential Lateral Solidification (SLS) techniques, as described in Robert S. Sposilli, M. A. Crowder, and James S. Im, Mat. Res. Soc. Symp. Proc. Vol. 452, 956.about.957, 1997. The SLS technique uses a phenomenon wherein the grain boundaries in directionally solidified materials tend to form perpendicularly to the melted interface. The SLS technique enables the conversion of as-deposited amorphous or polycrystalline silicon films into a directionally solidified microstructure consisting of long, columnar grains.
The laser beam pattern for using SLS technique is prepared by an annealing apparatus as shown in FIG. 2. An unpatterned laser beam is emitted from a light source 20 and is passed through an attenuator 22 to control the energy density of the unpatterned laser beam. The unpatterned laser beam is focused on a focus lens 22 and passed through a mask having a predetermined pattern 23 in order to pattern the laser beam. The patterned laser beam then passes through an imaging lens 24. A film 29 on a translation stage 25 is irradiated by the patterned laser beam. The entire film is scanned by the laser beam at a predetermined repetition rate. In this regard, Mirrors 28-1, 28-2, and 28-3 control the path of the laser beam.
FIGS. 3A to 3C show a method for crystallizing an amorphous silicon film by the SLS technique according to another related art. Referring to FIG. 3A, a narrow region having a slit film shape, bounded by the dashed lines 42 and 43, is irradiated at an energy density sufficient to induce complete melting. Subsequently, lateral grain growth proceeds from the unmelted regions to the adjacent narrow strip region 41 which is fully-melted. Grain boundaries in directionally solidified materials proceed perpendicularly to the melt interface. Depending on the width of the molten region, lateral growth ceases when either of two events occur: (1) the two opposing growth fronts collide at the center, or (2) the molten region becomes sufficiently supercooled to cause bulk nucleation of solids.
Due to these restrictive events, the maximum lateral growth distance which can be achieved with a single pulse is limited depending on the film thickness and the incident energy density.
Referring to FIG. 3B, the film is translated relative to the beam image over a distance less than the single-pulse lateral growth distance and irradiated again. Lateral growth begins again from the edges of the completely molten region, located within the grains grown during the previous irradiation step. The length of the grains is increased beyond the single-pulse lateral growth distance. For example, a narrow region 45 bounded by dashed lines 46 and 47 is irradiated by a second laser pulse. Since one of these edges, in this case the edge 46, is located within the silicon region grown during the previous irradiation step, the lengths of the silicon grains formed by the previous irradiation are extended by the second irradiation beyond the single-phase lateral growth distance.
Referring to FIG. 3C, the above-described processes of irradiation and solidification can be repeated indefinitely, creating grains of any desired length. The final resultant microstructure is shown.
Using the method of the related art, a polycrystalline silicon film having uniform physical characteristics could be achieved through lateral growth of the silicon grain. However, the polycrystalline silicon film cannot be used to form devices for complicated circuits, whereas a single crystal silicon film may be use in the manufacture of such devices.
FIGS. 4A to 4C illustrate a method for crystallizing an amorphous silicon film using the SLS technique according to a third related art. Referring to FIG. 4A, a selected region having a chevron-shaped aperture of the film and bounded by the dashed lines is irradiated at an energy density sufficient to induce complete melting. Subsequently, lateral grain growth proceeds from the unmelted region to the adjacent narrow strip, fully-melted region. The grain boundaries in directionally solidified materials tend to form perpendicularly in relation to the melt interface.
Because of the above-noted considerations, the maximum lateral growth distance that can be achieved with a single pulse is limited depending on the film thickness and the incident energy density. More specifically, the grain formed at the apex of the chevron experiences lateral growth not only in the translation direction, but also transverse to it, because the grain boundaries are formed perpendicularly to the melt interface. Therefore, the negative curvature of the molten zone at the apex of the chevron leads to a widening of the grain, such that a single crystal silicon grain region is induced.
Referring to FIG. 4B, the film is translated relative to the beam image over a distance which is approximately one-half of the single-pulse lateral growth distance, and irradiated again. Lateral growth recommences from the edges of the completely molten region, located within the grains grown during the previous irradiation step. The length of the grains is increased beyond the single-pulse lateral growth distance. The grain formed at the apex of the chevron, a single crystal silicon region, widens.
As shown in FIG. 4C, the above-cited processes of irradiation and solidification can be repeated indefinitely, resulting in grains of any desired length. The final resultant microstructure is illustrated. As shown in FIG. 4C, the grain formed at the apex of the chevron, a single crystal silicon region, widens dramatically.
FIG. 5 shows TEM data, indicating a crystalline silicon film crystallized by using the chevron-shaped laser beam. This drawing shows a polycrystalline silicon region, having lateral columnar silicon grains grown perpendicular to the interface between the amorphous silicon region and the irradiated silicon region, and a single crystalline region formed from the apex of the chevron to its upper portion.
However, defects in the sub-boundaries areas can be found in the single crystalline region. Therefore, the crystallized silicon films formed by using a chevron-shaped laser beam do not yield TFT devices which can achieve the level of performance of silicon wafer-based TFT devices.
FIG. 6 shows the distribution of the energy density of an irradiated silicon film portion by a chevron-shaped laser beam. The energy density of the other portions, except the apex portion of the chevron-shaped beam, shows a normal distribution, the brim of which is low and the center is high. But the energy density of the apex portion 51 of the chevron-shaped film is low relative the other portion which surrounds it. The explanation lies in the fact that the interference of the laser beam occurs at the apex because of the characteristics of the chevron-shaped beam. Thus, differences of the thermal stress can form in the molten silicon portion irradiated by the apex of the laser beam. The principal defect lies in the interior of the crystallized silicon.